Transformers for switched mode power supplies and other similar applications are becoming more and more critical as operating frequencies rise and as agency safety requirements become more stringent.
Leakage inductance becomes more and more critical as frequency rises. The cores should become smaller with increasing frequency, in theory, but may actually become larger, in practice, due to flux derating. Proximity effects become severe, and overheating is a serious problem.
The matrix transformer solves many of these problems. Please see U.S. Pat. Nos. 4,665,357; 4,845,606, 4,942,353; 4,978,906, 5,093,646 and 5,479,146, which describe various embodiments of the matrix transformer, and which are incorporated herein by reference.
Reference is also made to a tutorial entitled "Design and Application of Matrix Transformers and Symmetrical Converters" by the present inventor and published May 11, 1990.
The matrix transformer works particularly well for switched mode power supplies having a low voltage, high current output. The matrix transformer uses fewer primary turns, because the number of elements used is a factor in the equivalent turns ratio. That is, the equivalent turns ratio is the product of the number of primary turns and the number of elements (modules) used to one. This relationship assumes a one turn secondary winding. Often, a one-turn push pull secondary winding (alternatively called a two-turn, center-tapped winging) is used, to directly drive a dual rectifier.
For low power applications, where, from a power perspective, fewer elements could be used, the number of primary turns can still be quite high, particularly when operated from a high voltage source. This can make the matrix transformer difficult to wind and also reintroduces some of the problems of conventional transformers.
Agency safety requirements, though very necessary, are not at all helpful in designing transformers for switched mode power supplies and like applications. Thicker insulation is required, with more layers and spacing, all of which increase the bulk of the winding and increase its leakage inductance and thermal impedance.
Where shock hazard is a concern, interwinding capacitance must be kept very low. Interwinding capacitance can also conduct EMI from the input to the output and vice versa.
This invention addresses many of these problems.